In recent years, with the development of electronic instruments, it has been desired to develop small-sized, light electrochemical devices which have a high energy density and can be repeatedly charged or discharged. In particular, the use of electric double layer capacitors which have a long lifetime and can give a large electric current has been increasing by making use of advantages thereof.
Demands for electric double layer capacitors, in which an electric double layer formed in an interface between a polarizable electrode and an electrolyte is utilized, as backup power sources have been rapidly increasing in recent years. Attention has been paid to the application thereof to articles for which a large capacity is required, such as a power source for an electric vehicle.
In order to obtain an electrode for an electric double layer capacitor, there is used a binder such as a polytetrafluoroethylene, a phenol resin, cellulose or water glass. The electrode obtained by use of these binders is insufficient in binding force between its active material for an electrode and its current collector since the electrode is poor in flexibility. In the case of using a conventional acrylic resin or polyvinyl alcohol as the binder, the resin covers the surface of activated carbon, which is an active material for an electrode, and the resin itself is easily dissolved in an electrolytic solution. It is therefore impossible to keep the binding force between the activated carbon and the current collector sufficiently. For this reason, the electric resistance in the resultant electrode becomes high and the electrostatic capacity of the resultant electric double layer capacitor is low.
Suggested is also a method for improving the capacity of an electrode obtained by use of a binder containing a styrene-butadiene copolymer (Patent Document 1). However, when this binder is used to apply activated carbon onto a current collector and then the resultant is dried at high temperature to remove water content therefrom, the flexibility of the electrode is lost so that the activated carbon falls off easily from the current collector. Consequently, a highly-reliable electrode for an electric double layer capacitor cannot be obtained. Patent Document 1: Japanese Patent Application Laid-Open No. 11-162794